y1=k1x+b1与x轴的夹角a1满足tana1=k1
y2=k2x+b2与x轴的夹角a2满足tana2=k2
tan(a1-a2)=(tana1-tana2)/(1+tan1*tan2)
当k1*k2=-1,tan(a1-a2)=无穷
a1-a2=π/2,
y1与y2相互垂直